1. Field of the Invention
Embodiments of the present invention relate to a novel method and apparatus for measuring fluid flow velocity using a cooled, dual-element, thermocouple-based sensor.
2. Related Art
At low flow velocities, conventional, low cost flow-measuring techniques, such as differential pressure meters, target meters, or turbine meters, have poor performance. Thermal flowmeters or thermal xe2x80x9canemometers,xe2x80x9d however, have much better performance at low flow velocities. Nearly all previous work in the field of thermal anemometry has involved a resistive heating element to create a thermal driving force. Those thermal anemometers that pass electric current through a resistive wire are known as hot-wire anemometers, and those that pass electric current through a resistive film are known as hot-film anemometers. These traditional thermal anemometers require flow-sensing probes of specialized construction. Such probes are often delicate and increase the initial cost, and the maintenance and repair cost, of the instruments.
U.S. Pat. No. 4,848,147 (Bailey) for a Thermal Transient Anemometer, issued to Bailey and Josip in July, 1989, discloses a method of measuring fluid flow velocity which may use a cooled thermocouple. However, upon comparison, it will be notable to those skilled in the art that the present invention is different from Bailey in several ways. The Bailey method specifically calls for a delay in temperature measurement to purposely avoid measuring initial probe conditions. Bailey calls for this delay to xe2x80x9callow the initial probe conditions to become negligible,xe2x80x9d and then measures temperature at only two later points in time. In addition, Bailey utilizes no data or regression validation during flow measurement.
Additional prior art references are discussed in my Thesis entitled xe2x80x9cUse of a Thermoelectrically Cooled Dual Element Thermocouple For Fluid Velocity Measurement,xe2x80x9d which is part of the provisional application specifically incorporated herein by reference.
Still, there is a need for a more effective cooled thermocouple velocity measurement method. A measurement method according to the principles of the present invention does not suffer the performance and accuracy limitations of the prior art, as is further discussed below.
The present invention comprises use of a cooled thermocouple and improved calibration and correlation methods to measure low velocities of fluid flow. The thermocouple is preferably cooled by the Peltier effect during application of a substantially constant direct electrical current. As is well-known, the Peltier effect can be used either to heat or to cool depending on the direction of the electric current. The real advantage of the Peltier effect, especially in this application, is that it can be used to cool the thermocouple junction to a temperature lower than the fluid flowing past it. Many real-world applications exist where cooling the probe is more desirable than heating the probe. This is particularly true when the fluid being measured is a liquid at or near the boiling point. When a liquid is at the boiling point, a heated probe will produce a phase change at the probe surface, and when this occurs, the probe is insulated by a vapor phase boundary and is no longer able to sense the moving liquid.
Dual element thermocouples are constructed in a large variety of configurations and materials, and manufacturers produce them to withstand a variety of harsh environments commonly encountered in industrial applications. The preferred flow-measuring device of this invention uses a low cost, readily-available dual element thermocouple, such as a dual common junction ungrounded thermocouple, for the flow sensing probe. The preferred device is expected to be effective up to flowrates of about 100 mm/second.
In the present invention, the junction of the thermocouple is calibrated and positioned in a flowing fluid. A known electric current is applied through one pair of conductors in the dual element thermocouple. The electric current flowing through the junction causes Peltier heat to be absorbed, thereby cooling the thermocouple junction. The amount of Peltier heat removed from the junction is a function of the electric current flow and temperature of the junction. By pulsing the electric current on and off, the junction will alternately cool and then warm to the temperature of the surrounding bulk fluid.
Cooling of the junction constitutes a thermal driving force that results in heat being transferred between the fluid and the thermocouple junction. The unit area heat transfer depends on the size of the temperature difference and the heat transfer coefficient between the fluid and the thermocouple junction. Between current pulses, therefore, one may monitor the response of the junction to this heat transfer, which is related to the heat transfer coefficient that is in turn related to the fluid velocity.
Monitoring of the junction temperature response is done, in the preferred embodiment, by using the second thermocouple pair of the preferred dual element thermocouple to measure the actual temperature of the junction. The temperature response of the junction is measured immediately after the current is shut off and also while the thermocouple junction warms to the bulk fluid temperature. Therefore, the temperature data of greatest interest may be summarized as follows:
1. The xe2x80x9cinitial conditionxe2x80x9d temperature of the probe is measured, wherein xe2x80x9cinitialxe2x80x9d refers to the temperature of the probe after it has reached a cool state due to the Peltier effect. Preferably the cool state is measured at the time the junction has reached a first equilibrium, but this requirement, may be relaxed somewhat as long as a sufficiently long time has passed. The cool junction temperature is measured and compared to the bulk fluid temperature (see item 3 below) in a term described as the observed xe2x80x9ctemperature suppression,xe2x80x9d (Tbulkxe2x88x92Tprobe (at time zero)) Therefore, the initial condition temperature preferably is measured immediately after the current is shut off. xe2x80x9cImmediatelyxe2x80x9d, in this context, means within preferably {fraction (1/60)} of a second or less.
2. The temperature of the junction at a plurality of points in time after the current is shut off is also measured. Preferably, many of these xe2x80x9ctemperature curve pointsxe2x80x9d are measured (at least two in addition to the initial condition temperature from item 1 above) while the probe is heating up to reach a second equilibrium at the temperature of the surrounding fluid.
3. The temperature of the bulk fluid is measured for input into the temperature suppression calculation. The bulk fluid temperature may be measured, by the same probe that measures the xe2x80x9cinitial conditionxe2x80x9d and xe2x80x9ccurve pointxe2x80x9d data, by taking a junction temperature measurement after the probe junction has reached the second equilibrium (with the Peltier current off.) at the temperature of the bulk fluid, thus allowing use of a single probe for the all the data collection. Alternatively, the bulk fluid temperature may be measured by a separate thermocouple probe immersed in the flowing fluid, in which case the requirement of reaching the first equilibrium may be somewhat relaxed as long as sufficient temperature curve points have been obtained. Either way, this bulk fluid temperature is periodically observed and stored, and preferably is observed and stored for each new temperature suppression calculation.
This data, comprising xe2x80x9cinitial conditionxe2x80x9d data, xe2x80x9ctemperature curve point dataxe2x80x9d, and bulk fluid temperature, is acquired during initial probe calibration in known fluid velocities and is used to obtain probe calibration constants. Then, for each flow velocity measurement, this data is observed during normal probe operation in the unknown fluid velocities of interest for calculation of the observed temperature suppression term (Tbulkxe2x88x92Tprobe (at time zero)) and for regression analysis to obtain the observed time constant. The observed temperature suppression and time constant are input, along with the probe calibration constants stored in memory, into a computer algorithm to ultimately compute the fluid flow velocity. In summary, a calibration equation is developed relating the junction""s temperature suppression and observed time constant to the fluid velocity.
Therefore, it is an object of the present invention to provide a method of measuring fluid flow velocity that is accurate and economical, using a cooled, dual-element, thermocouple sensor. It is another object to provide a method of accurately measuring fluid flow velocity in fluids that are moving at low velocities, for example, at less than 100 mm/second, and, most effectively, at about 0-30 mm/s. It is another object of the present invention to provide a method of accurately measuring fluid flow velocities in fluids that are near their boiling-point.
The present invention accomplishes some or all of the above objects with features that differentiate it from the prior art. The invented method uses no delay after a power pulse, but rather uses immediate sampling of temperature data, that is, the method measures initial conditions immediately after the pulse. The present invention specifically uses this information from the observed initial conditions to provide a more accurate measure of the fluid flow velocity. The present invention makes use of repeated temperature measurements over time (herein called xe2x80x9ctemperature curve pointsxe2x80x9d) to determine the observed time constant of the thermocouple. Although as few as two xe2x80x9ctemperature curve pointxe2x80x9d measurements (after the initial condition measurements) may be used, the preferred number is in the range of approximately 30-180. This measurement approach allows the present invention to use both time constant data and steady state temperature suppression to correlate to fluid velocity. The preferred invention also includes validation of the data observations and/or the regression analysis during each flow velocity measurement. As a result of these improved features, the invention is well-suited to detect other system changes (for example, changing bulk fluid temperature, or changing thermocouple signal noise) that can lead to erroneous computations.
Other objects, features and advantages of the present invention will become apparent from the following description.